Current radiation protection standards are based on the premise that even low dose of ionizing radiation may result in detrimental health effects, such as long-term development of cancer and hereditary disease. Cardiovascular testing accounts for at least one-third of all ionising testing and invasive cardiologists working in a busy catheterization laboratory are the most exposed among professionally exposed physicians. Interventional cardiologists have an exposure per-head per year two to three times higher than that of radiologists. Chromosome aberration (CA)-based assays are considered the gold standard of biological dosimeter of radiation exposure and intermediate endpoints of cancer risk. We and others have recently reported that cardiologists working in a high-volume catheterization laboratory chronically exposed to low-dose radiation well within allowed limits show increased chromosomal damage versus non-exposed controls. However, from a case-control study, it is not completely possible to determine the role of other potentially confounding factors that appear to have great influence on the frequency of CA, such as age, smoking or individual genetic susceptibility. In this study, features of chromosomal damage were studied in a monozygotic male twin pair, exploiting an unique model to study how genetically identical individuals can exhibit differences due to the role of different environmental factors.